As Treasurer Joe Hockey weighs the arguments over the takeover bid of GrainCorp, one of the oldest farm organisations in the US has warned that increased concentration in the industry will be bad news for Australian farmers.

The National Farmers Union says it has watched the negative impact of foreign takeovers in its own country, and says Australia should reject the deal offered by Archer Daniels Midland (ADM).

But the American growers who deal daily with the company say it is the only way to make the most of a booming global demand.

In Decatur, Illinois, the presence of ADM in grain-producing areas is hard to to miss. The company's name can be seen for kilometres on silos across the rural landscape.

Its processing plant at its global headquarters in Decatur operates 24 hours a day, seven days a week.

Farming group warns about competition impact

In this part of the US, people are well aware of ADM's bid for GrainCorp.

Analysis - Correspondent Lisa Millar

Archer Daniels Midland is a global giant. But there is something of a small town feel to its base in Decatur.

The closest major airport is two to three hours away. Drive for 10 minutes out of this rural city and you're in sparse countryside, dotted with silos. These prairie plains of Illinois offer the kind of soil farmers dream about.

The company says it'll leave the processing plants here but next year it's moving its headquarters somewhere more convenient. This is a business on the move in more ways than one.

The ABC was waved through security at the current headquarters like welcomed guests. But our two-hour visit came with plenty of restrictions.

We could only film the outside of the buildings. Trucks, trains and flags flapping in the stiff Illinois wind are the images you'll see on the ABC.

Anything inside the company's processing plants is off limits. A request for an interview was turned down on the basis the spokespeople are currently in Australia talking up the deal. Our media escort – the sole public relations officer for ADM in Decatur - was extremely pleasant and helpful. But she wasn't letting us out of her sight.

ADM might have been deflecting the questions on US soil, but it's not hard to find others happy to talk. From local farmers to university professors to Washington-based lobbyists, they all know this company well. Not everyone's a fan.

ADM's tentacles spread far and its power is no secret.

The US National Farmers Union is firm in its belief that consolidation and concentration in Australia isn't good news for growers but it took several phone conversations and a conference call hook-up to get them on camera.

And then there was David Brown, a grain grower who spoke highly of ADM and was so technologically connected he put us to shame. He tweeted to other farmers about our visit before we'd even left his property.

Don't be in any doubt, they're all watching what's happening in Australia closely, even if their allegiances differ.

Chandler Goule from the National Farmers Union, which includes small and medium-sized farmers among its members, thinks the Australian Government should think twice before approving the takeover bid.

"Any time a group of companies, or one or two companies, corner a market and infiltrate it by 40 to 60 per cent, economists will tell you you've lost competition," he said.

"And as you lose competition, then what happens to your producers is they're limited on who they can sell to, therefore they get a lower price and your end users are limited on who they can purchase from.

Mr Goule predicts ADM will be the only winner out of the deal.

"You don't want the increased concentration," he said.

"ADM is a very legitimate, successful company. Really what we're looking at here is the economic factors that are associated with increased concentration and with increased vertical integration in your agriculture industry.

"We're not focused on the company, but on the economic factors and the negative consequences associated with increased concentration."

Patrick Woodall from the Washington-based lobby group Food and Water Watch says farmers in Australia will be played off against farmers in South America, where ADM also has a presence.

"The global sourcing of commodities allows them to exert downward pressure on prices worldwide because they can pit farmers against one another," he said.

"They're going to buy a certain amount of wheat. Now they're in more places where wheat is grown and they can just push down on prices."

Crossbench MPs apply pressure

The warning comes as a group of federal crossbench MPs have team up in an effort to put pressure Treasurer not to approve the deal.

Clive Palmer, Andrew Wilkie and Bob Katter, as well and the Greens' Adam Bandt, are planning to move a private members bill in the lower house to give Parliament a say on the issue.

Mr Palmer says he expects Nationals MPs in the Coalition to speak out against the sale when the bill is debated.

"It'll give every member of the National Party a chance to be counted so that their electorate knows they have a vote, whether or not GrainCorp should be sold, or whether they should be betrayed," he said.